1. Field of the Invention
This invention relates to a fine positioning device suitable for use in an apparatus which requires fine adjustment on the order of micrometers, such as semiconductor fabrication apparatus or electron microscope, and also to a displacement controller for such a fine positioning device.
2. Description of the Prior Art
In various fields of technology, there has recently been a strong demand for devices which enable fine adjustment of displacements on the .mu.m order. As a typical technical field, may be mentioned semiconductor fabrication apparatus employed in the fabrication processes of LSI (large-scale integrated circuits) and super large-scale integrated circuits, such as mask aligners, electron-beam drawing machines and the like. In these apparatus, it is necessary to achieve fine positioning of the .mu.m order. The degree of integration increases and products of higher performance can be fabricated, as the positioning accuracy is improved. Such fine positioning is required not only for the above-described semiconductor fabrication apparatus but also for a variety of high-magnification optical apparatus, led by electron microscopes, and the like. Improved accuracy contributes significantly to the development of advanced technology such as biotechnology, space development, etc.
As such fine positioning devices, a variety of devices have heretofore been proposed, for example, as shown in a Japanese magazine, "Kikai Sekkei (Machine Designing)", 27(1), 32-36, January 1983. Of such fine positioning devices, those making use of parallel springs and fine motion actuators are considered to be superb in that inter alia, they do not require cumbersome displacement reducing mechanisms and their structures are simple. Accordingly, a fine positioning device of the above sort will hereinafter be described with reference to FIG. 1.
FIG. 1 is a side view of a conventional fine positioning device, in which there are illustrated a support table 1, planar parallel springs 2a, 2b fixed in parallel to each other on the support table 1, and a fine motion table 3 supported on the parallel spring 2a, 2b and having a high degree of rigidity. Designated at numeral 4 is a fine motion actuator mounted between the support table 1 and fine motion table 3. The fine motion actuator 4 makes use of a piezoelectric element, electromagnetic solenoid or the like, which is energized to apply a force to the fine motion table 3 along the x-axis of the coordinate system depicted in the figure.
The parallel springs 2a, 2b, reflecting their structure, have low rigidity in the direction of the x-axis but high rigidity in the direction of the z-axis and in the direction of the y-axis (namely, in the direction perpendicular to the drawing sheet). When the fine motion actuator 4 is energized, the fine motion table 3 thus undergoes a displacement practically along the x-axis only and no substantial displacements take place in the other directions.
FIG. 2 is a perspective view of another conventional fine positioning device which is readily conceivable from the devices disclosed by way of example in the above-mentioned magazine. In the figure, there are shown a support plate 6, a pair of 20 planar parallel springs 7a, 7b fixed in parallel to each other on the support table 6, a middle table 8 fixed on the parallel springs 7a, 7b and having a high degree of rigidity, another pair of parallel springs 9a, 9b fixed on the middle table 8 and extending in parallel to each other in a direction perpendicular to the parallel spring 7a, 7b, and a fine motion table 10 fixed on the parallel spring 9a, 9b and having a high degree of rigidity. When a coordinate system is established as shown in the figure, the parallel spring 7a, 7b are arranged along the x-axis, while the parallel springs 9a, 9b are disposed along the y-axis. This structure corresponds basically to a structure obtained by stacking two structures, each of the same type as the one-axis (displaceable along the x-axis only) structure depicted in FIG. 1, one over the other. An arrow F.sub.x indicates a force to be applied along the x-axis to the fine motion table 10, while an arrow F.sub.y designates a force to be applied along the y-axis to the middle table 8. Unillustrated actuators which can apply the forces F.sub.x, F.sub.y are provided respectively between the support table 6 and fine motion table 10 and between the support table 6 and the middle table 8.
When the force F.sub.x is applied to the fine motion table 10, the parallel springs 9a, 9b are deformed. Since the parallel springs 7a, 7b have high rigidity against the force F.sub.x applied along the x-axis, the fine motion table 10 is allowed to undergo a displacement practically along the x-axis only. When the force F.sub.y is exerted to the middle table 8 on the other hand, the parallel springs 7a, 7b are deformed and by way of the parallel springs 9a, 9b, the fine motion table 10 is displaced practically along the y-axis only. When both forces F.sub.x, F.sub.y are applied at the same time, the parallel springs 7a, 7b, 9a, 9b are simultaneously deformed. Correspondingly, the fine motion table 10 is displaced two-dimensionally.
As described above, the device shown in FIG. 2 can perform positioning along two axes whereas the device illustrated in FIG. 1 is a one-axis positioning device.
In each of the devices shown respectively in FIGS. 1 and 2, the fine motion table 10 is displaced linearly along the specific axis. On the other hand, Japanese Patent Publication No. 50433/1982 discloses a fine positioning device in which a fine motion table is caused to undergo a fine angular displacement about a specific axis. This fine positioning device will next be described with reference to FIG. 3.
FIG. 3 is a partially cut-away perspective view of a conventional fine positioning device which makes use of fine angular displacements. In the figure, numeral 11 indicates a fixed central portion in the form of a cylindrical column and numerals 11a, 11b, 11c designate vertical slots formed with an equal interval, along the length of the fixed central portion, in the circumferential wall of the fixed central portion 11. There are also depicted a ring-shaped stage 12 provided movably about the fixed central portion 11 and U-like metal members 12a.sub.1 -12a.sub.3, 12b.sub.1 -12b.sub.3, 12c.sub.1 -12c.sub.3 secured fixedly on the stage 12 in opposition to the vertical slots 11a, 11b, 11c respectively. Designated at numeral 13 are bimorph cells mounted between the individual vertical slots 11a, 11b, 11c and their corresponding U-like metal members 12a.sub.1 -12c.sub.3, while numeral 13A indicates beads fixed on the bimorph cells 13 at locations where the bimorph cells 13 engage their corresponding U-like metal members 12a.sub.1 -12c.sub.3. The fixed central portion 11, stage 12 and individual U-like metal members 12a.sub.1 -12c.sub.3 are all rigid. Here, the above-mentioned bimorph cells 13 are described in brief with reference to FIG. 4.
FIG. 4 is a perspective view of one of the bimorph cells 12a.sub.1 -12c.sub.3. In the figure, there are shown piezoelectric elements 13a, 13b and a common electrode 13c provided between the piezoelectric elements 13a, 13b. The piezoelectric elements 13a, 13b are rigidly cemented together with the common electrode 13c interposed therebetween. Designated at numerals 13d, 13e are surface electrodes applied fixedly to the piezoelectric elements 13a, 13b respectively. In the above stacked or double-layered structure, when a voltage of such a polarity that the piezoelectric element 13a is caused to contract is applied between the surface electrode 13d and common electrode 13c and at the same time, another voltage of such a polarity that the piezoelectric element 13b is caused to expand is applied between the surface electrode 13e and common electrode 13c, the piezoelectric elements 13a, 13b are respectively caused to contract and expand in the directions shown by arrows. As a result, the bimorph cell 13 is deformed as a whole as shown in the figure. Owing to this property, the bimorph cell 13 can provide a greater degree of displacement compared with a single piezoelectric element.
In the device shown in FIG. 3, the bimorph cells 13 which have the above-described property are fixed at one ends thereof in their corresponding vertical slots 11a, 11b, 11c but the other ends of the bimorph cells 13 remain as free ends and are kept via their respective beads 13A in contact with the corresponding U-like metal members 12a.sub.1 -12c.sub.3. Let's now assume that suitable voltages are applied respectively to the bimorph cells 13 so as to cause them to undergo such deformations as shown in FIG. 4. Corresponding to the deformations of the bimorph cells 13, the stage 12 undergoes an angular displacement about the fixed central portion 11. If a fine motion table is fixedly mounted on the stage 12, it is possible to have the fine motion table to undergo the angular displacement.
In the above-described conventional device, the U-like metal members 12a.sub.1 -12c.sub.3 and their corresponding bimorph cells 13 are kept in mutual contact. Owing to this structural feature, the bimorph cells are mounted while being allowed to undergo free deformations. This structural feature can thus avoid the imminent restraint (interference) of displacements which takes place if the bimorph cells 13 should be fixed to the stage 12.
The fine positioning devices depicted in FIGS. 1 and 2 may be used only for one-dimensional and two-dimensional positioning respectively. They can produce neither displacement along the z-axis nor angular displacement about the x-, y- or z-axis. Turning to the fine positioning device shown in FIG. 3, it can produce neither displacement along the x-, y- or z-axis nor angular displacement about any one of the other two axes. From these conventional fine positioning devices, it may only be feasible to contemplate a 3-axis fine positioning device, which can produce displacements along both x-axis and y-axis and an angular displacement about the z-axis, by combining the device of FIG. 2 and that of FIG. 3 together. It is believed to be extremely difficult to construct a 4-axis or larger multi-axis fine positioning device on the basis of such conventional devices.